Adds `napi_set_instance_data()` and `napi_get_instance_data()`, which
allow native addons to store their data on and retrieve their data from
`napi_env`. `napi_set_instance_data()` accepts a finalizer which is
called when the `node::Environment()` is destroyed.
This entails rendering the `napi_env` local to each add-on.
Fixes: nodejs/abi-stable-node#378
PR-URL: #28682
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>